Great question, and great debate. I have wrestled with this over the years and ended up with both (612 standard and 615 softened). Not sure which one I like better, honestly, but have been gaming the 612 most recently.

Given this, I'm sure what the answer is for you other than "personal preference". Honestly, I think it's more an issue of WIDTH than edges/rounding. You can have Byron adjust either to whatever cornering and edges you want (Tai treatment, etc.), but the 615 is definitely wider (see pics below that I shared in a thread about a year ago). And the width difference in the top-lines is significant if that's important to you (it is to me)....

Agree....I actually think it really comes down to topline width. I have had both and your eye will really go to that topline...if you like the thinner topline go 612, thicker 615.

+1 on what was said above. Also would add the 612 may be a bit more refined milled piece but have putted with both. I like the added width of the 615 and looks of the 612 and both feel very good. If it were me I would opt for the less expensive 615 and have topline and bumpers rounded and all edges softened (Tai Treatment)

All of this agreed- the last piece for me is the sole- the 615 is a tri sole, and the 612 a radius sole.Depending on the greens you are playing, and how you sole your putter, this could make a difference.I will say that all the above being said, for me the 612 is a bit more demanding of my stroke, where the 615 I don't feel I need to concentrate as much.My 615 is about 4 grams heavier on the head than my 612.RC

Lots of subtle differences between the two but for me the two big, noticeable differences are the overall size and the neck position (if you're going with a stock, plumbers neck).

The 615 has a much larger footprint. It's a wider and (I think) slightly longer putter than the 612 and it's top line is also a bit thicker.

The neck position is totally different. The look and how the putter releases feels very different too. The neck on the 612 is nearly flush with the face. The neck sits much further back from the face on the 615.

I have a 612 and an old 006 (615 predecessor) and will probably order a 615 at some point. Like you OP, I don't like the sharp bumpers on the new 615 so I'd probably have Byron round those off. Even though functionally, I prefer my 006 to my 612, the 612 does look a bit more refined of a design.

In my mind, it's all about the sole. Do you love or hate a tri-sole. For me, I feel constricted with a tri sole, so when the time comes, I'll be going 612. Some guys love the tri sole because it gives them a sense comfort knowing that the putter is sitting on the ground consistently every time.